Dentin microstructure and properties are determinants of nearly all operations in restorative dentistry. Normal dentin exhibits variation in structure and properties with location, but among the various additional dentin types, the altered forms of carious coronal dentin and sclerotic root dentin are particularly important. They result from vital defensive mechanisms associated with the odonoblasts which alter composition, hardness and permeability. These altered forms of dentin are important substrates for dentin bonding. Thus progress in adhesion to dentin and dentin physiology require a thorough knowledge of their basic microstructure, composition, and properties. This project will extend the knowledge and techniques developed ina th prior period for normal dentin to these two important variations of dentin which are encountered in many clinical situations. Caries produces a multi-layered structure in dentin with varying degrees of structural loss, demineralization and hypermineralization. Traditionally the deepest altered layer is termed transparent dentin, due to its altered refractive index or sclerotic dentin to reflect the increase in hardness due to increased mineralization. However, in some cases, transparent dentin is softer than normal dentin. This suggests that varying subclasses of transparent dentin could result from caries and suggests that the use of transparent and sclerotic dentin as interchangeable terms may not be correct. Sclerotic dentin of the root is much less studied, and is considered to form due to a variety of irritation processes in the cervical region and aging throughout the root, even in the absence of caries and is characterized by occlusion of the tubule lumen with additional crystals that are acid resistant, at least in cervical lesions. Alteration of the intertubular dentin has not been established. it is clear that there is insufficient information on the structure and properties of these altered dentins and their particular zones. The central hypothesis to be tested is that the structure of transparent carious dentin and sclerotic non-carious root dentin exhibit important structural, compositional, and properties differences as compared with normal dentin. We will test this hypothesis and clarify the structure and properties of these altered forms of dentin through conduct of ten specific aims, five related to caries and five parallel aims related to sclerotic cervical and root dentin. These include mapping zones of altered dentin including the mineral level by XTM, structure by SEM/EDS and AFM, hardness by microhardness and AFM nano-hardness (aims 1,6). We also will test the hypothesis that altered forms of dentin respond differently to demineralization and exhibit changes in permeability (aims 2,6). The hypothesis that the composition and structure of the intertubular zone mineral is altered, while the peritubular is not, will be tested for carious dentin using micro-FTIR, AFM, SEM/EDS/WDS and TEM with electron diffraction and PEELS (aims 3,4). Similar studies will be conducted for sclerotic cervical and root dentin using similar methods (aims 8,9).